Geotechnical analysis of dolomitic terrain to assess the probability of sinkhole formation : a case study of Centurion, South Africa

Abstract

Dolomitic terrain in the Gauteng Province of South Africa presents significant geotechnical challenges for urban development. This is primarily due to subsurface dissolution processes that create void networks susceptible to sinkhole formation and ground subsidence. This research study explores the case of Cornwall Hill in Centurion where numerous sinkhole incidents have caused infrastructure damage and imposed severe development constraints. A multi-method investigation approach was employed, combining gravity geophysical surveys, test pit excavations, rotary percussion boreholes, dynamic cone penetrometer tests, and laboratory analysis. Gravity surveys comprised 105 measurement stations on a 50 m × 50 m grid. Five test pits were excavated while four rotary percussion boreholes were drilled to depths between 10 and 25 m. Five dynamic cone penetrometer tests were also carried out on site and rotary percussion drilling was done to complement gravity surveys. Lastly, a comprehensive laboratory geotechnical analysis programme was done on field samples collected on site. Results revealed that the site is underlain by Malmani Subgroup dolomites overlain by a consistent three-layer stratigraphic sequence comprising colluvium, residual soil, and dolomite bedrock. The residual soils exhibited consistent classification as low plasticity clay with plasticity indices of 11 – 14% and a gravel-rich composition at 50 – 60%. However, the soils displayed significant spatial variability in thickness (0.70 – 2.70 m) and bearing capacity (152 – 320 kPa at 0.7 – 0.8 m depth). Rotary percussion drilling confirmed the presence of shallow competent dolomite beneath chert layers at 2 m depth and colluvium overlying interbedded chert and dolomite from 3 m depth. A cavity located within the peripheral low-density gravity zone and extending from 2 m to 13 m depth before competent material was also reached. Gravity survey results defined the subsurface density contrast, with high-density areas (covering 60% of the site) indicating shallow competent bedrock at 1.0–1.6 m depth and low-density areas (covering 40% of the site) indicating deep weathering profiles exceeding 2.7 m; the cavity intersected in BH01 directly correlates with low gravity in the peripheral zone. The site was described into Zone A (central D3 area of the site suitable for development with raft foundations at 600–900 mm depth) and Zone B (peripheral D4 area of the site unsuitable for development owing to the presence of a cavity, gravity lows indicating solution features, and high collapse potential) based on the results of integrated geophysical, intrusive, and laboratory investigation. The study meets the requirements of SANS 1936-2:2012 and proves the potential of integrated characterization of complex terrain, including the application of Rotary Percussion Drilling (RPD) as ground truth, for making informed decisions regarding development of difficult terrain such as this dolomitic environment, while providing valuable geotechnical data for the Centurion region.